(1) Field of the Invention
The present invention relates generally to methods of communicating, and more particularly to a method of communicating in a time-varying medium of transmission (e.g., seawater) when only one sensor is available for reception.
(2) Description of the Prior Art
It is established that performance of jointly adaptive, multichannel decision feedback equalizers is superior to that of single-channel decision feedback equalizers. That is, if the signal from a single source is received at multiple sensors, application of a jointly adaptive equalizer to the data from the multiple sensors simultaneously is superior to applying an adaptive equalizer to the data received from a single sensor.
The theory and description of multisensor/multichannel adaptive equalization (as it is known) are described in detail in several prior art references. See, for example, Stojanovic et al., "Coherent Communications Over Long Range Acoustic Telemetry Channels," NATO ASI Series on Acoustic Signal Processing for Ocean Exploration, pp. 607-612, 1993; Stojanovic et al., "Adaptive Multichannel Combining and Equalization of Underwater Acoustic Communications," J. Acoust. Soc. Amer., Vol. 94, No. 3, Pt. 1, pp. 1621-1631, Sept. 1993; Catipovic et al., "Spatial Diversity Processing For Underwater Acoustic Telemetry," IEEE Journal of Oceanic Engineering, Vol. 16, No. 1, pp. 86-97, Jan. 1991; and Jarvis et al., "Implementation of a Multichannel Decision Feedback Equalizer for Shallow Water Acoustic Telemetry Using a Stabilized Fast Transversal Filters Algorithm," Proceedings of Oceans '95, October 1995.
These references assume that the data to be telemetered is bundled into a finite and relatively short duration packet (or packets as is the case for a long data message) and transmitted one time. Also, the assumption is made that multiple spatially separated (or spatially diverse) sensors are available to receive the data. The underlying principle of the existing, prior art method is that a single transmission travels through independent paths in order to reach the spatially separated sensors. Then, in a manner analogous to beamforming, the output of the spatially separated sensors can be combined to recover more of the signal than is possible with a single sensor alone.
Many high-rate telemetry systems use multichannel adaptive equalization to process the telemetered data. As stated above, multichannel adaptive equalization is known to be superior to single channel adaptive equalization. However, for many communications applications such as underwater acoustic telemetry, often only one sensor is available. This is especially true in many Navy applications where the receiving platform has already been designed and deployed. The addition of acoustic telemetry is often a "back fit" to an existing system. Where only one receiving sensor is available, the telemetry receivers/processors incorporated into the existing systems cannot currently take advantage of multichannel adaptive equalization processes.